An offshore drilling rig in the Gulf of Mexico is on fire. Regulators say the rig has partially collapsed. It all began yesterday when members of a drilling crew lost control of a natural gas well they were completing. The crew was evacuated from the rig as a cloud of gas escaped into the air. And then last night, the gas caught fire. NPR's Jeff Brady tells us more.

JEFF BRADY, BYLINE: The drilling rig is in relatively shallow water - 154 feet deep, about 55 miles off the coast of Louisiana. Ryan Tippets with the Coast Guard says everyone is being kept away.

RYAN TIPPETS: Right now, the Coast Guard is enforcing a safety zone around the burning rig. It's 500 meters by five miles.

BRADY: The Coast Guard and the U.S. Bureau of Safety and Environmental Enforcement were able to fly over the rig today. A huge ball of flames continues to engulf much of the rig, but the agencies report no sign of sheen or oil in the water.

The companies involved are Walter Oil & Gas Corporation and rig owner Hercules Offshore, both based in Houston. The companies say they're busy working with regulators to bring the well under control and were not available for interviews. Bud Danenberger worked as an offshore drilling regulator for nearly 40 years and now is a consultant.

BUD DANENBERGER: Fire certainly complicates things. And they're going to need to stop that before they can pursue any type of response from the surface.

BRADY: Danenberger says even if crews were able to extinguish the fire, gas will still be flowing from the well, which will create an ongoing safety hazard. He says the next step likely will be to drill a relief well nearby. That would be used to intercept the flow of gas that's currently feeding the fire. The crew could then plug the well. But drilling that relief well could take weeks.

The Bureau of Safety and Environmental Enforcement says the drilling rig and offshore platform have started to collapse. Eric Smith with the Tulane Energy Institute in New Orleans says that's to be expected.

ERIC SMITH: Whenever you have the kind of extreme heat that's associated with a gas fire offshore, eventually, the structural steel support members that hold the rig together start sagging and melting.

BRADY: Smith says a rig like this one has several backup systems in place to keep a well under control, including a blowout preventer. Unlike BP's Deepwater Horizon accident in 2010, this blowout preventer is above the water and on the rig. Smith says in theory, that should have made it easier to deploy the blowout preventer in an emergency.

SMITH: Even if all the electronics fail and all the hydraulics fail, people can literally turn big wheels and close the shear rams and shut off the flow. One of the interesting questions will be why that wasn't successful in this particular case.

BRADY: And Smith says there likely will be a full investigation to determine what led to this blowout and the fire, which continues to burn in the Gulf of Mexico. Jeff Brady, NPR News.